1. Field of the Invention
The present invention relates to a multilayer piezoelectric device and a piezoelectric actuator including such a multilayer piezoelectric device.
2. Description of the Related Art
In general, a piezoelectric actuator which makes use of a piezoelectric effect is sometimes used in a print head of an inkjet printer. As shown in FIG. 8, a piezoelectric actuator 1o has a piezoelectric property which causes it to be displaced in what is called a d31 direction, that is, a direction that is perpendicular to the thickness direction of a piezoelectric device. A portion of the bottom surface of the piezoelectric actuator 1o is adhered to a fixation substrate 2, and a pressure-generating chamber 3 is disposed in contact with the end portion of the piezoelectric actuator 1o that is not adhered to the fixation substrate 2. One end of a signal-inputting flexible printed circuit board (hereinafter referred to as xe2x80x9cFPCxe2x80x9d) 4 is connected to the top surface of the piezoelectric actuator 1o. Accordingly, when a signal is input from the FPC 4, the piezoelectric actuator 1o is displaced in a direction that is perpendicular to the thickness direction thereof, causing pressure to be exerted onto ink inside the pressure-generating chamber 3, so that the ink is discharged outside the pressure-generating chamber 3.
The structure shown in FIGS. 9 and 10 has been known as a structure of a multilayer piezoelectric device 6o included in a piezoelectric actuator 1o. (For example, refer to Japanese Unexamined Patent Application Publication No. 11-10875.)
More specifically, the related multilayer piezoelectric device 6o includes a rectangular piezoelectric substrate 7. Short internal electrodes 8 and long internal electrodes 9, having different lengths, are arranged so that the short internal electrodes 8 extend from a side surface 7b towards an opposite side surface 7c and the long internal electrodes extend from the side surface 7c to the opposite side surface 7b. In addition, the short internal electrodes 8 and the long internal electrodes 9 are alternately stacked in the piezoelectric substrate 7. The side surfaces 7b and 7c are opposite each other in the direction of a short side of the piezoelectric substrate 7. In other words, in the direction of a long side (longitudinal side) of the piezoelectric substrate 7, the electrodes 8 and 9 have lengths that are substantially equal to the overall width of the piezoelectric substrate 7. On the other hand, in the direction of a short side of the piezoelectric substrate 7, the short internal electrodes 8 are shorter than the long internal electrodes 9. Active portions 10 which are displaced in a direction that is perpendicular to the lamination direction are located at the portions where the electrodes 8 and the corresponding electrodes 9 overlap. Therefore, as shown in FIG. 10, the active portions 10 are adjacent to the left side surface 7b of the two side surfaces 7b and 7c that are opposite each other in the direction of a short side of the piezoelectric substrate 7, and have shapes that are asymmetrical in the horizontal direction.
Each of the short internal electrodes 8 is in a drawn-out state at the side surface 7b of the piezoelectric substrate 7 near the active portions 10, and each of the long internal electrodes 9 is in a drawn-out state at the side surface 7c of the piezoelectric substrate 7 distant from the active portions 10. A first external electrode 13o and a second external electrode 14o are electrically connected to the drawn-out short internal electrodes 8 and the drawn-out long internal electrodes 9, respectively.
The first external electrode 13o extends from the side surface 7b of the piezoelectric substrate 7 near the active portions 10 towards the side surface 7c and distant from the active portions 10 by extending onto a main surface 7a, and terminates before reaching the side surface 7c. On the other hand, the second external electrode 14o extends from the side surface 7c of the piezoelectric substrate 7 and distant from the active portions 10 towards a location which opposes an end portion of the first external electrode 13o at a predetermined interval d therefrom by extending onto the main surface 7a. The certain interval d between the first and second external electrodes 13o and 14o is provided because it is necessary to electrically isolate them.
As described later, when forming the piezoelectric actuator 1o using the multilayer piezoelectric device 6o, electrical conductors on the FPC 4 need to be electrically connected to the first and second external electrodes 13o and 14o. Therefore, an area which extends a predetermined distance of L4 from the side surface 7c of the piezoelectric substrate 7 and distant from the active portions 10 is provided as a predetermined soldering portion 16o required for soldering.
Such a multilayer piezoelectric device 6o and the piezoelectric actuator 1o including the multilayer piezoelectric device 6o, are formed, for example, in the following way.
In order to form the multilayer piezoelectric device 6o, as shown in FIG. 11, three types of green sheets 31, 32, and 33 are provided. In other words, there are provided the green sheets 31 which have electrically conductive patterns 41, which define the short internal electrodes 8, disposed thereon. The green sheets 32 which have electrically conductive patterns 42, which define the long internal electrodes 9, disposed thereon, and the green sheets 33 which do not have any electrically conductive patterns formed thereon, are also provided. These green sheets 31, 32, and 33 are produced by forming piezoelectric materials, such as PZT materials, into rectangular shapes in plan view. The electrically conductive patterns 41 and 42 are formed by screen printing electrically conductive paste whose main component is, for example, silver (Ag).
After alternately and sequentially placing upon each other a predetermined number of each of the green sheets 31 and 32 having the corresponding electrically conductive patterns 41 and 42 formed thereon, the green sheets 33 are disposed on the topmost and bottommost portions of the resulting layered structure. Then, all of these sheets 31, 32, and 33 are press-bonded all together along the lamination direction, and baked. After the baking, the first and second external electrodes 13o and 14o are formed by evaporation. Thereafter, by polarizing the piezoelectric substrate 7, the multilayer piezoelectric device 6o including the active portions 10 at the portions where the short internal electrodes 8 and the corresponding long internal electrodes 9 are stacked is produced.
Next, using the multilayer piezoelectric device 6o, formed in the above-described manner, the piezoelectric actuator 1o is formed as shown in FIG. 12. First, the multilayer piezoelectric device 6o is adhered to the fixing substrate 2 with an adhesive so that its active portions 10 do not overlap the fixing substrate 2.
Then, using a dicing saw, cuts of predetermined sizes are formed at predetermined pitches along the longitudinal direction of the multilayer piezoelectric device 6o from the side surface 7b towards the side surface 7c in order to form slits 21. As a result, movable portions 22o which can be separately driven are formed between the slits 21. Further, cuts are formed in the first external electrode 13o consecutively with the corresponding slits 21 in order to form shallow grooves 23, so that the first external electrode 13o is electrically isolated from each movable portion 22o.
Next, the FPC 4 is soldered onto the predetermined soldering portion 16o, so that the electrical conductors on the FPC 4 are electrically connected to each of the divided first external electrode 13o portions and the second external electrode 14o. For example, signal-input-side electrical conductors on the FPC 4 are connected to the corresponding divided first external electrode 13o portions, and a ground-side electrical conductor is connected to the second external electrode 14o. Pressure-generating chambers 3 are separately disposed at the corresponding movable portions 22o of the piezoelectric actuator 1o. Accordingly, by separately inputting signals to each of the divided first external electrode 13o portions, the movable portions 22o are separately driven, so that ink from each of the pressure-generating chambers 3 is separately discharged.
As mentioned above, in the multilayer piezoelectric device 6o included in the piezoelectric actuator 1o, it is necessary to electrically connect the FPC 4 to both electrodes 13o and 14o while the end portions of the first and second external electrodes 13o and 14o at the main surface 7a oppose each other at the predetermined distance d from each other. Therefore, in the related multilayer piezoelectric device 6o, a relatively large length L4 extending from the side surface 7c of the piezoelectric substrate 7 and distant from the active portions 10 is required as the predetermined soldering portion 16o. As a result, the predetermined soldering portion 16o is disposed close to the active portions 10.
In producing the piezoelectric actuator 1o, when the FPC 4 is soldered onto the predetermined soldering portion 16o, the FPC soldering location is close to the active portions 10, so that, not only is smooth expansion and contraction of each of the movable portions 22o prevented, but also the following problems arise due to transmission of vibration, produced by displacement of the movable portions 22o, to other portions through the FPC 4.
More specifically, when a signal Vp having an applied voltage waveform such as that represented by waveform a of FIG. 13 is input to a predetermined movable portion 22o for a period of time of t1 to t2, if the FPC-4-soldering location is sufficiently separated from the active portion 10, the vibration is damped within a short period of time, as shown by waveform b of FIG. 13. In contrast, when the FPC-soldering location is close to the active portion 10, the vibration damping time becomes longer, as shown by waveform c of FIG. 13. As a result of this, ink discharge becomes unstable, thereby causing reduced image quality.
In addition, when the soldering location is close to the active portion 10, the active portion 10 loses its polarization property because it is affected by heat produced during soldering, thereby causing, for example, the problem that active portion 10 becomes unable function piezoelectrically.
To solve these problems, the predetermined soldering portion 16o may be moved further away from the locations of the active portions 10 by, for example, making long a distance L5 from the side surface 7b of the piezoelectric substrate 7 near the active portions 10 to the predetermined soldering portion 16o in the multilayer piezoelectric device 6o. However, when this is done, an overall length L3 of the piezoelectric substrate 7 in the direction of a short side thereof becomes long so that, not only is it impossible to reduce the size of the entire multilayer piezoelectric device 6o, but also, for example, costs of the multilayer piezoelectric device 6o are increased. Therefore, it is not advisable to make the overall length L3 long.
In order to overcome the above-described problems, preferred embodiments of the present invention provide a multilayer piezoelectric device which is greatly reduced in size and which has an excellent piezoelectric property as a result of making it possible to provide a sufficient distance between an active portion and a soldering location when a piezoelectric actuator is constructed. In addition, other preferred embodiments provide a piezoelectric actuator including such a novel multilayer piezoelectric device.
According to a preferred embodiment of the present invention, a multilayer piezoelectric device including a substantially rectangular piezoelectric substrate, long internal electrodes, and short internal electrodes. The long internal electrodes and the short internal electrodes have different lengths and arranged so that the long internal electrodes and the short internal electrodes extend from a pair of opposing side surfaces of the piezoelectric substrate towards the corresponding side surfaces of the piezoelectric substrate opposite to the side surfaces from which the long internal electrodes and the short internal electrodes extend. The long internal electrodes and the short internal electrodes are also arranged such that the short internal electrodes and the long internal electrodes are alternately stacked inside the piezoelectric substrate. The multilayer piezoelectric device further includes active portions which are disposed at portions where the long internal electrodes and the corresponding short internal electrodes are stacked, and which are displaced in a direction that is substantially perpendicular to a direction in which the long internal electrodes and the corresponding short internal electrodes are stacked, a first external electrode which is electrically connected to the short internal electrodes at the drawn-out side surface, and a second external electrode which is electrically connected to the long internal electrodes at the drawn-out side surface. The first external electrode and the second external electrode are separately formed on a surface of the piezoelectric substrate. In addition, the first external electrode extends from the side surface of the piezoelectric substrate near the active portions, extends onto a main surface, and extends to a location adjacent to the side surface distant from the active portions where the long internal electrodes are drawn out. The second external electrode extends onto the main surface so as to be disposed on left and right sides of locations at the first external electrode that are adjacent to the side surface of the piezoelectric substrate distant from the active portions.
The second external electrode may be arranged so as to extend onto the main surface from the side surface of the piezoelectric substrate distant from the active portions. Alternatively, the second external electrode may be arranged so as to extend onto the main surface from a side surface of the piezoelectric substrate that is substantially perpendicular to the side surface of the piezoelectric substrate distant from the active portions.
According to preferred embodiments of the present invention, unlike a related multilayer piezoelectric device where the end surfaces of the first and second external electrodes are face and oppose each other, the multilayer piezoelectric device of the present invention is constructed so that the second external electrode is disposed on both the left and right sides of locations at the first external electrode that are adjacent to the side surface distant from the active portions. Therefore, it is possible to dispose each of the first and second external electrodes adjacent to the side surface of the piezoelectric substrate and distant from the active portions. Therefore, the location where the FPC is to be soldered no longer needs to be situated very far from the side surface of the piezoelectric substrate distant from the active portions, so that the location where the FPC is to be soldered can be sufficiently separated from the active-portion-formation locations. As a result, comparing the multilayer piezoelectric device of preferred embodiments of the present invention with a related multilayer piezoelectric device, the overall length of the piezoelectric substrate in the direction of a short side thereof can be made shorter. In addition, the entire multilayer piezoelectric device of preferred embodiments of the present invention can be made much smaller than a related multilayer piezoelectric device.
According to another preferred embodiment of the present invention, a piezoelectric actuator wherein, by forming slits, extending from the side surface near the active portions towards the side surface opposite thereto of the multilayer piezoelectric device of any one of the preferred embodiments of the present invention described above, at a predetermined pitch along a direction that is substantially perpendicular to the direction of extension of the slits, separately drivable movable portions are provided between the corresponding slits, and an electrical conductor on a flexible printed circuit board is connected to the first external electrode, divided by the slits, and the second external electrode.
In the piezoelectric actuator, since the FPC-soldering location is disposed away from the active portions, the movable portions expand and contract smoothly, and transmission of vibration, produced by displacement of the movable portions, to other portions through the FPC is reliably prevented. Therefore, vibration is damped within a short period of time, so that ink is stably discharged, thereby providing excellent image quality. In addition, since the soldering portion is disposed away from the active portions, there is no possibility of the polarization properties of the active portions being deteriorated by heat produced during soldering, so that stable piezoelectric properties are provided.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.